/*
Given a linked list, swap every two adjacent nodes and return its head.

For example,
Given 1->2->3->4, you should return the list as 2->1->4->3.

Your algorithm should use only constant space. You may not modify
 the values in the list, only nodes itself can be changed.
*/

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
  public ListNode swapPairs(ListNode head) {
        if (null == head || null == head.next) {
            return head;
        }
        ListNode node = head.next;
        head.next = swapPairs(node.next);
        node.next = head;
        head = node;
        return head;
    }

}